Room-Temperature C–C σ-Bond Activation of Biphenylene Derivatives on Cu(111)

Activating the strong C–C σ-bond is a central problem in organic synthesis. Directly generating activated C centers by metalation of structures containing strained four-membered rings is one maneuver often employed in multistep syntheses. This usually requires high temperatures and/or precious transition metals. In this paper, we report an unprecedented C–C σ-bond activation at room temperature on Cu(111). By using bond-resolving scanning probe microscopy, we show the breaking of one of the C–C σ-bonds of a biphenylene derivative, followed by insertion of Cu from the substrate. Chemical characterization of the generated species was complemented by X-ray photoemission spectroscopy, and their reactivity was explained by density functional theory calculations. To gain further insight into this unique reactivity on other coinage metals, the reaction pathway on Ag(111) was also investigated and the results were compared with those on Cu(111). This study offers new synthetic routes that may be employed in the in situ generation of activated species for the on-surface synthesis of novel C-based nanostructures.

aqueous NH 4 OH (15 mL). The organic layer was separated and the aqueous phase was extracted with ethyl acetate (2 x 20 mL). The combined organic layers were dried and concentrated under reduced pressure. From the resulting suspension, nitrobenzene was removed by careful decantation, and the solid was washed with cold hexane (2 x 20 mL) and dried under vacuum, to afford 2,2',6,6'-tetrabromobiphenyl (II) (740 mg, 38%). 1

STM Experiments
STM measurements were performed using a commercial Scienta-Omicron LT-STM at 4.3 K.
The system consists of a preparation chamber with a typical pressure in the low 10 -10 mbar regime and a STM chamber with a pressure in the 10 -11 mbar range. The Cu(111) and Ag(111) crystals were cleaned via two cycles of Ar + sputtering and annealing (720 K for Cu and 700 K for Ag). All STM and STS measurements were performed at either at 78 K or 4.3 K. To obtain BR-STM images, the tip was functionalized with a CO molecule that was picked up from the metal surfaces. CO was deposited onto the sample via a leak valve at a pressure of approximately during which the samples warmed up only to about 270 K before being put back into the cryogenic STM environment. For the room temperature annealing experiment, the sample was taken out from the cold STM head and kept at the sample storage for a few hours. A similar procedure was used for the control experiment involving the biphenylene molecule whose evaporation temperature is 350 K and deposition time is 2 minutes.

XPS Experiments
X-ray photoelectron spectroscopy has been carried out holding the sample at room temperature and illuminating it with monochromatized Al Ka light from a SPECS µ-FOCUS 600 setup. The of the C1s peak is nearly identical before and after annealing to 600K. This means that no desorption of carbon atoms is detected. For the Br 3p core level, the fit gave a spin orbit splitting between the 3p 3/2 and 3p 1/2 components of 6.7 eV and an intensity ratio of 3:1. The leading 3p 3/2 line was detected at 182.0 eV binding energy. Like for C 1s also in Br 3p no intensity drop after annealing was observed. Differences in the noise level are related to different acquisition times (as in the C 1s spectra), but the intensities in counts per second remain similar, implying that the isolated Br stays on the surface rather than desorbing from it.

Computational Details of DFT Calculations
All slab density functional theory (DFT) calculations used the PBE exchange-correlation For the dimers, we used an enlarged 6x6x4 slab (144 metal atoms, 36 atoms per layer).
All initial geometries were constructed using the ASE module (version 3.21.1). 4 Over 15 angstroms of vacuum layer and dipole corrections were used to decouple the periodic images along the normal z direction.
We performed the slab calculations within the projector-augmented wave method (PAW) 5 as implemented in the code GPAW (version 21.1.0). 6,7 For the GPAW calculations, we used the plane wave (PW) mode with a converged kinetic energy cutoff of 500~eV. We used an electronic Fermi-Dirac smearing temperature of 0.1 eV, and a reciprocal space K-point mesh of 3x3x1. With this PW computational setup, already used in Ref. 8